U-PB ZIRCON AGE AND ND ISOTOPIC COMPOSITION OF GRANITOIDS, CHARNOCKITES AND SUPRACRUSTAL ROCKS FROM HEIMEFRONTFJELLA, ANTARCTICA

Heimefrontfjella and Mannefallknausane, in Dronning Maud Land, Antarctica, comprise an amphibolite-facies terrain and a granulite terrain, separated by a major mylonite zone. The amphibolite terrain is made up of mafic to felsic metavolcanics and metasediments, intruded by granitoid plutons: the granulite terrain has supracrustal rocks with similar lithologies, intruded by felsic plutonic rocks that crystallized as charnockites. U-Pb zircon ages (conventional and ion microprobe) demonstrate that magmatic activity was confined to a relatively short interval between 1130 and 1045 Ma and was followed in the amphibolite terrain by metamorphism around 1060 Ma. Specific ages are as follows: metarhyolite in the amphibolite terrain, 1093 +/- 38 Ma; granitoids in the amphibolite terrain, 1045 +/- 9 Ma to 1107 +/- 16 Ma, charnockites in the granulite terrain, 1073 +/- 8 Ma to 1135 +/- 8 Ma, metamorphic zircons in garnet amphibolite and a post-metamorphic pegmatite, both 1060 +/- 8 Ma. Older zircons were found only in a metasediment which yielded discordant zircon fractions with Pb-207/Pb-206 ages between approximately 1250 and 1450 Ma, and in a granulite facies metaquartzite, which contained concordant zircons with the following ages: 1104 +/- 5 Ma, 1215 +/- 15 Ma, approximately 1400 Ma, approximately 1700 Ma, approximately 2000 Ma. The youngest age is interpreted as the age of granulite metamorphism, the older ages as those of detrital zircons. A Sm-Nd mineral isochron age of the garnet amphibolite (960 +/- 120 Ma) agrees within error with the U-Pb age of metamorphic zircons (1060 +/- 8 Ma). Initial epsilon-Nd values (T = 1.1 Ga) for 15 samples range from +4 to -4. The highest came from a metabasalt and two granitoids from Milorgfjella, the northern area; the lowest from the granulite-facies metasediment and from a charnockite, both from Vardeklettane, a nunatak in the south. The positive but subdued values preclude generation directly from depleted MORB-type mantle (epsilon-Nd approximately +6 to +7 at 1.1 Ga) and indicate generation from a source containing older crustal material.